1. Field of the Invention
The present invention relates to a high-frequency semiconductor device, i.e., a high-frequency transistor, in which collector, emitter, and base electrodes are formed on a major surface of a substrate and, more particularly, to a high-frequency semiconductor device incorporated in an IC chip and used in a wide-band amplifier.
2. Description of the Related Art
Conventionally, a high-frequency transistor in which collector, emitter, and base electrodes are formed on a major surface of a substrate has a schematic two-dimensional pattern shown in FIG. 1. FIG. 2 is a sectional view showing the high-frequency transistor along a line II--II in FIG. 1. Referring to FIG. 2, reference numeral 1 denotes a p-type semiconductor substrate; 2, an n.sup.+ -type buried collector layer; 3, an n-type collector layer; 4, an n.sup.+ -type collector extraction layer; 4a, a collector electrode; 5, a p-type base layer; 6, a p.sup.+ -type base layer; 6a, a base electrode; 7, an n.sup.+ -type emitter layer; 7a, an emitter electrode; 8, a thin film resistor; and 9, a transistor region.
The n.sup.+ -type buried collector layer 2 is formed in the transistor region 9. The n.sup.+ -type collector extraction layer 4 (continuous layer) reaching the major surface of the substrate is formed at both the ends of the n.sup.+ -type buried collector layer 2. The p-type base layer 5 is formed in a surface region of the n-type collector layer 3 defined by the n.sup.+ -type buried collector layer 2 and the n.sup.+ -type collector extraction layer 4, and the p.sup.+ -type base layers 6 and the n.sup.+ -type emitter layers 7 are alternately formed one by one in the surface region of the base layer 5. The collector electrode (continuous layer) 4a is formed on the n.sup.+ -type collector extraction layer 4. The base electrode (continuous layer) 6a is formed on the p.sup.+ -type base layer 6. The emitter electrode (continuous layer) 7a is formed on the n.sup.+ -type emitter layer 7. The emitter electrode 7a is connected to an external emitter electrode 7b through the emitter resistor 8 consisting of a thin film resistor.
The base electrode 6a extends from only one end of the transistor region 9 in relation to the pattern of an electrode layer and a wiring layer. The collector electrode 4a extends from both ends of the transistor region 9.
Because the base electrode 6a extends from only one end of the transistor region 9, the collector current is increased to increase a transition frequency, i.e., improve high-frequency characteristics, the base resistance is increased, and the transistor has poor stability. That is, in order to increase the transition frequency, a pitch between the p.sup.+ -type base layer 6 and the n.sup.+ -type emitter layer 7 must be decreased. When the pitch between the p.sup.+ -type base layer 6 and the n.sup.+ -type emitter layer 7 is decreased, the capacity therebetween is decreased, thereby improving high-frequency characteristics. However, since electrode widths of the base electrode 6a and the emitter electrode 7a are also decreased, the current density of a current flowing through these electrodes is increased. As the result, a collector current is increased, and the base resistance is increased. Therefore, the circuit operation becomes unstable. In order to solve the above problem, a decrease in high-frequency output and a decrease in high-frequency gain must be considered.
As described above, in the conventional semiconductor device, since a base electrode extends from only one end of a transistor region, as the collector current is increased, the base resistance is increased, and the operation becomes unstable. In order to solve the above problem, high-frequency output and high-frequency gain is necessarily decreased.